1. Technical Field
The present invention relates generally to electronic circuits, and more specifically, the invention relates to switched mode power supplies.
2. Background Information
A typical requirement for power supplies of electronic equipment is that they limit their output power. One reason to limit output power is to meet the requirements of safety agencies for prevention of personal injury. Another reason to limit output power is to avoid damage to electronic components from an overload.
Power supplies typically have self-protection circuits that respond when an output becomes unregulated for a specified time. However, if output power is not limited, a fault at a load can consume enough power to cause damage or to exceed regulatory requirements while the outputs remain regulated. Thus, the self-protection feature can be ineffective if the power supply can deliver too much power.
A common way to limit output power of a switching power supply is to limit the current in a power switch at the input of the power supply. The maximum output power is related to the peak current in the switch. Inherent delays in the responses of electrical circuits create an error between the desired limit for peak current in the switch and the actual maximum peak current in the switch. The error is greater at higher input voltages, causing the maximum output power to be greater at higher input voltages than it is at lower input voltages.